Gelled polymers and methods of preparing same

ABSTRACT

Improvements in secondary recovery operations for the recovery of oil, and improvements in well drilling operations, are accomplished through the use of aqueous mediums comprising aqueous gels prepared from strong brines and certain polyacrylamides and related polymers.

United States Patent Hessert et al.

14 1 Sept. 30, 1975 1 1 GELLED POLYMERS AND METHODS OF PREPARING SAME[75] Inventors: James E. Hessert; Richard L.

Clampitt, both of Bartlesville, Okla.

[73] Assignee: Phillips Petroleum Company,

Bartlesville, Okla.

[22] Filed: May 17, 1973 [21] Appl. No.: 361,236

Related US. Application Data [62] Division of Ser. No. 224,915, Feb. 9,1972, Pat. No.

[52] US. Cl 252/855 D; 166/270; 166/275; 252/316; 252/855 R; 252/855 A;252/855 C [51] Int. Cl. E21B 43/16 [58] Field of Search 252/316, 8.55 R,8.55 D, 252/855 A, 8.55 C; 175/65, 72; 166/294, 295, 270, 275

[56] References Cited UNITED STATES PATENTS 3,020,953 2/1962 Zcrweck et11 252/855 Primary ExaminerBenjamin R. Padgett Assistant Ii.\'an1iner-B.Hunt 5 7 1 ABSTRACT Improvements in secondary recovery operations forthe recovery of oil, and improvements in well drilling operations, areaccomplished through the use of aqueous mediums comprising aqueous gelsprepared from strong brines and certain polyacrylumides and relatedpolymers.

22 Claims, No Drawings GELLED POLYMERS AND METHODS OF PREPARING SAMEThis application is a division of our copending application Ser. No.224,915, filed February 9, 1972, now US. Pat. No. 3,749,172.

This invention relates to methods of preparing and using aqueous gelsprepared from polyacrylamides and related polymers.

The secondary recovery of oil from oil-bearing or containingsubterranean formations by fluid drive pro-v cesses wherein a fluid isinjected into the formation via one or more injection wells to drive theoil through the formation to one or more production wells is a wellknown process. Fluids used in such processes include liquids, such aswater and various hydrocarbons, and gases such as hydrocarbon gases,carbon dioxide, etc. Many oil reservoirs comprise layers or zones ofporous rock which can vary in permeability from more than 1,000millidarcys to less than millidarcys. In all fluid drive processes arecognized problem is the predilection of the drive fluid to channelalong or through the more permeable zones of the formation. This iscommonly referred to as fingering. The more conductive zones, after theoil has been largely displaced therefrom, function as thief zones whichpermit the drive fluid to channel directly from injection to productionwells. In many instances, such channeling or fingering results inleaving substantial quantities of oil in the less permeable zones of theformation which are bypassed. Such channeling or fingering can occurwhen the mobility, i.e., the quotient of the reservoirs permeability tothe drive fluid divided by the viscosity of the drive fluid, becomeslarge relative to the mobility of the reservoir oil.

Drilling fluids used in the drilling of oil wells, gas wells, andsimilar boreholes are commonly aqueous liquids containing clays or othercolloidal materials. The drilling fluid serves as a lubricant for thebit and drill stern, as a carrying medium for the cuttings produced bythe drill bit, and assists in the formation of a filter cake on the wallof the borehole for the reduction of fluid losses to the surroundingsubsurface strata. It is known that excessive viscosity in the drillingfluid has an adverse effect on the penetration rate obtained by thedrill bit. In many instances, substantially better rates can be securedby eliminating colloidal materials and reducing the viscosity of thedrilling fluid. In some instances, air, clear water, or other similarfluid of low viscosity can be used in the place of the ordinary drillingfluid or mud.

It has been discovered-that certain aqueous gels can comprise at least aportion of the aqueous medium used in said secondary recoveryoperations, and the aqueous medium used in said well drillingoperations. Said gels are prepared from certain water-dispersiblepolymers, e.g., polyacrylamides and other related polymers, which whenused in combination with a water-soluble compound of a polyvalent metalwhich can be reduced to a lower polyvalent valence state and a suitablereducing agent capable of reducing said polyvalent metal to said lowerpolyvalent valence state, can be used as gelling agents to gel aqueousmediums comprising water. By varying the composition and/or amounts ofsaid gelling agents, and/or the conditions under which they are used informing the gels, a wide range of aqueous gels ranging from liquidhighly mobile gels to thick, viscous, somewhat elastic gels can beproduced. Said aqueous gels are particularly useful in operationswherein a fluid mediumis introduced into a borehole in the earth,'e.g.,in the above-described secondary op erations, in the above-describedwell drilling operations, in well completion operations, as packerfluids,

etc. I

In preparing such aqueous gels for use in oil field operations itisdesirable for economic and other reasons to use water which is readilyavailable in the field. Frequently, the only readily available water isfield brine, produced from wells in the'field, and containing largeamounts of total dissolved solids. However, it has not been possible touse such brines with the desired assurance of success. In manyinstances, for reasons not heretofore known, it has been impossible toobtain gels, or when gels are obtained, to obtain gels having therequired stability. In order to be'assured of success it has beennecessary to use fresh water, or at least water containing a relativelysmall amount of total dissolved solids. In many instances the cost ofobtaining such low solids content water can be almost prohibitive.

The present invention provides a solution for the above-describedproblem. We have now discovered that when using brines containing largeamounts of total dissolved solids, the polyacrylamide or related polymerwhich is used should be one wherein not more than about 14, preferablynot more than about 12, percent of the carboxamide groups have beenhydrolyzed to carboxyl groups.

The use of aqueous gels prepared from such polymers is particularlyadvantageous in near-well treatments of nonfractured porous formations.The characteristics of higher levels of adsorption by the sands of theformation leads to a longer lasting water diversion effect which can bedemonstrated by higher residual resistance factors for longer periods oftime.

Another advantage in using strong field produced brines in preparingaqueous gels as described herein is that the problem of disposing ofsuch brines is lessened.

Thus, according to the invention, there is provided, in a method whereina fluid medium is introduced into a borehole in the earth and intocontact with a nonfractured porous subterranean formation penetrated bysaid borehole, the improvement wherein at least a portion of said fluidmedium comprises an aqueous gel, and wherein said gel comprises water towhich there has been added: a water-thickening amount of awaterdispersible polymer selected from the group consisting of:polyacrylamides and polymethacrylamides wherein from 0.1 to about 14percent of the carboxamide groups are hydrolyzed to carboxyl groups;crosslinked polyacrylamides and crosslinked polymethacrylamides whereinfrom 0.1 to about 14 percent of the carboxamide groups are hydrolyzed tocarboxyl groups; copolymers of acrylamide with another ethylenicallyunsaturated monomer copolymerizable therewith, sufficient acrylamidebeing present in the monomer mixture to impart said water-dispersibleproperties to the resulting copolymer when it is mixed with water, andwherein from 0.1 to about 14 percent of the carboxamide groups arehydrolyzed to carboxyl groups; and mixtures of said polymers; a sensibleamount of a water-soluble compound of a polyvalent metal wherein themetal present iscapable of being reduced to a lower polyvalent valencestate and which is sufficient to gel said water when the valence of atleast a portion of said metal is reduced to said lower valence state;and an amount of a water-soluble reducing agent which is effective toreduce at least a portion of said metal to said lower valence state; andwherein the initial total dissolved solids content of said water isgreater than about 60,000 ppm by weight.

Still further according to the invention, there is provided a method forproducing an aqueous gel, which method comprises: thickening water byadding thereto at least 0.1 weight percent, based on the weight of saidwater of a water-dispersible polymer selected from the group consistingof: polyacrylamides and polymethacrylamides wherein from O.l to about 14percent of the carboxamide groups are hydrolyzed to carboxyl groups;crosslinked polyacrylamides and crosslinked polymethacrylamides whereinfrom 0.1 to about l4 percent of the carboxamide groups are hydrolyzed tocarboxyl groups; copolymers of acrylamide with another ethylenicallyunsaturated monomer eopolymerizable therewith, sufficient acrylamidebeing present in the monomer mixture to impart said water-dispersibleproperties to the resulting copolymer when it is mixed with water, andwherein from 0.1 to about 14 percent of the carboxamide groups arehydrolyzed to carboxyl groups; and mixtures of said polymers; gellingthe sothickened water by adding .thereto an amount of a water-solublecompound of a polyvalent metal wherein the valence state of the metaltherein is capable of being reduced to a lower polyvalent valence stateand which is sufficient to supply at least about 3 X l gram atoms ofsaid polyvalent metal per gram of said polymer; and an amount of awater-soluble reducing agent which is effective to reduce at least aportion of said metal to said lower valence state; and then diluting theresulting gel with sufficient water to reduce the concentration of saidpolymer to a final desired concentration; the initial total dissolvedsolids content of said first-mentioned water being greater than about60,000 ppm by weight.

Herein and in the claims, unless otherwise specified, the term polymer"is employed generically to include both homopolymers and copolymers; andthe term water-dispersiblc polymers is employed to include thosepolymers which are truly water-soluble or brinesoluble and thosepolymers which are dispersiblc in water or other aqueous medium such asbrines to form stable colloidal suspensions which can be gelled asdescribed herein.

Polymers which can be used in the practice of the in vention include thevarious polyacrylamides and related polymers which are partiallyhydrolyzed, are water-dispersible, and which can be used in an aqueousmedium, e.g., a brine, with the gelling agents described herein, to givean aqueous gel. Presently preferred polymers include the varioussubstantially linear homopolymers and copolymers of acrylamide andmethacrylamide. By substantially linear it is meant that the polymersare substantially free of crosslinking between the polymer chains. Saidpolymers will have from 0.1 to about 14, preferably up to about 12,percent of the carboxamide groups hydrolyzed to carboxyl groups. As usedherein and in the claims, unless otherwise specified, the termhydrolyzed" includes modified polymers wherein the carboxyl groups arein the acid form and also such polymers wherein the carboxyl groups arein the salt form, provided said salts are water-dispersible. Such saltsinclude the ammonium salts, the alkali metal salts, and others which arewater-dispersible. Hydrolysis can be carried out in any suitablefashion, for example, by heating an aqueous solution of the polymer witha suitable amount of sodium hydroxide.

Substantially linear polyacrylamides can be prepared by methods known inthe art. For example, the polymerization can be carried out in aqueousmedium, in the presence of a small but effective amount of awatersoluble oxygen-containing catalyst, e.g., a thiosulfate orbisulfate of potassium or sodium or an organic hydroperoxide, at atemperature between about 30 and C. The resulting polymer is recoveredfrom the aqueous medium, as by drum drying, and can be subsequentlyground to the desired particle size. A presently preferred particle sizeis such that about weight percent will pass through a number 10 meshsieve, and not more than about l0 weight percent will be retained on a200 mesh sieve (U.S. Bureau of Standards Sieve Series).

Included among the copolymers which can be used in the practice of theinvention are the waterdispersible copolymers resulting from thepolymerization of a major proportion of acrylamide or methacrylamide anda minor proportion of an ethylenically unsaturated monomercopolymerizable therewith. It is desirable that sufficient acrylamide ormethacrylamide be present in the monomers mixture to impart to thecopolymer the above described water-dispersible properties, for example,from about 90 to 99 percent acrylamide and from about 1 to 10 percentother ethylenically unsaturated monomers. Such other monomers includeacrylic acid, methacrylic acid, vinylsulfonic acid, vinylbenzylsulfonicacid, vinylbenzenesulfonic acid, vinyl acetate, acrylonitrile, methylacrylonitrile, vinyl alkyl ether, vinyl chloride, maleic anhydride, andthe like. Various methods are known in the art for preparing saidcopolymers. For example, see US. Pat. Nos. 2,625,529; 2,740,522;2,729,557; 2,831,841; and 2,909,508. Said copolymers will be used in thehydrolyzed form, as discussed above for the homopolymers.

Crosslinked polyacrylamides and crosslinked polymethacrylamides,partially hydrolyzed as described above, can also be used in thepractice of the invention. in general, said crosslinked polyacrylamidescan be prepared by the methods described above, but

including in the monomeric mixture a suitable amount of a suitablecrosslinking agent. Examples of crosslinking agents includemethylenebisacrylamide, divinylbenzene, vinyl ether, divinyl ether, andthe like. Said crosslinking agents can be used in small amounts, e.g.,up to about 1 percent by weight of the monomeric mixture. Suchcrosslinking is to be distinguished'from any crosslinking which occurswhen solutions of polymers are gelled as described herein.

Mixtures of the above-described polymers can also be used in thepractice of the invention. All the polymers useful in the practice ofthe invention are characterized by high molecular weight. The molecularweight is not critical so long as the polymer has the above-describedwater-dispersible properties. It is preferred that the polymer have amolecular weight of at least 100,000. The upper limit of molecularweight is unimportant so long as the polymer is waterdispersible, andthe aqueous gel prepared therefrom can be pumped. Thus, polymers havingmolecular weights as high as 20,000,000 or higher, and meeting saidconditions, can be used.

The amount of said polymers used in the practice of the invention canvary widely depending upon the particular polymer used, the purity ofsaid polymer, and properties desired in said aqueous gels. In general,the amount of polymer used will be a water-thickening amount, i.e., atleast an amount which will significantly thicken the water or brine towhich it is added. For example, amounts in the order of 25 to 100 partsper million by weight (0.0025 to 0.01 weight percent) have been found tosignificantly thicken water. For example, distilled water containing 25ppm of a polyacrylamide having a molecular weight of about X l0 has aviscosity increase of about 41 percent. At 50 ppm the viscosity increaseis about 106 percent. At 100 ppm the viscosity increase is about 347percent. As another example, distilled water containing 25 ppm of apolyacrylamide having a molecular weight of about 3.5 X 10 has aviscosity increase of about 23 percent. At 50 ppm the viscosity increaseis about 82 percent. At 100 ppm the viscosity increase is about 24lpercent. Generally speaking, amounts in the range of from 0.0025 to 5,preferably from 0.01 to 1.5, more preferably 0.025 to 0.4, weightpercent, based on the weight of water or brine, can be used. However,amounts outside said ranges can be used. In general, with the properamounts of polyvalent metal and reducing agent, the amount of polymerused will determine the consistency of the gel obtained. Small amountsof polymer will usually produce liquid mobile gels which can be readilypumped whereas large amounts of polymer will usually produce thick,viscous, somewhat elastic gels. If desired, said thick gels can bethinned" by dilution with water or brine to any desired concentration ofpolymer. This can be done by mechanical means, e.g., stirring, pumping,or by means of a suitable turbulence inducing device to cause shearing,such as a jet nozzle. Thus, there is really no fixed upper limit on theamount of polymer which can be used.

However, we have discovered that when a liquid mobile gel is desired, itis definitely preferred to first prepare a concentrated gel and dilutethe more concentrated gels before they become too viscous. In general,dilute gels are more difficult to prepare in that, for one thing,gelling times are longer. More importantly for some reason not yetcompletely understood, the gels are usually more effective in theirintended uses when a concentrated gel is first prepared and then dilutedto the desired concentration. Another advantage is that, in general,less gelling agents are required for a given viscosity.

Metal compounds which can be used in the practice of the invention arewater-soluble compounds of poly valent metals wherein the metal ispresent in a valence state which is capable of being reduced to a lowerpolyvalent valence state. Examples of such compounds include potassiumpermanganate, sodium permanganate, ammonium ehromate, ammoniumdichromate, the alkali metal chromates, the alkali metal dichromates,and chromium trioxide. Sodium dichromate and potassium dichromate,because of low cost and ready availability, are the presently preferredmetal-containing compounds for use in the practice of the invention. Thehexavalent chromium in said chromium compounds is reduced in situ totrivalent chromium by suitable reducing agents, as discussedhereinafter. In the permanganate compounds the manganese is reduced from+7 valence to +4 valence as in MnO ill The amount of saidmetalcontaining compounds used in the practice of the invention will bea sensible amount, i.e., a small but finite amount which is more thanincidental impurities, but which is effective or sufficient to causesubsequent gelation when the metal in the polyvalent metal compound isreduced to a lower polyvalent valence state. The: lower limit of theconcentration of the starting metal-containing compound will depend uponseveral factors including the particular type of polymer used, theconcentration of the polymer in the water or brine to be gelled, thewater or brine which is used, and the type of gel product desired. Forsimilar reasons, the upper limit on the concentration of the startingmetal-containing compound also cannot always be precisely defined.However, it should be noted that excessive amounts. of the startingmetal compound, for example +6 chromium, which can lead to excessiveamounts of+3 chromium when there is sufficient reducing agent present toreduce the excess +6 chromium, can adversely affect the stability of thegels produced. As a general guide, the amount of the starting polyvalentmetal-containing compound used in preparing aqueous gels in accordancewith the invention will be in the range of from 0.05 to 60, preferably0.5 to 30, weight percent of the amount of the polymer used. Statedanother way, the amount of the starting polyvalent metal-containingcompound used will usually be an amount sufficient to provide at leastabout 3 X 10*, preferably at least 3 X 10*, gram atoms of said metalcapable of being reduced per gram of polymer. Preferably, the amount ofsaid metal capable of being reduced which is used will not exceed 4 X10", more preferably 2 X 10*, gram atoms of said metal per gram ofpolymer. However, in some situations it may be desirable to use amountsof the starting polyvalent metalcontaining compound which are outsidethe above ranges. Such use is within the scope of the invention. Thoseskilled in the art can determine the amount of starting polyvalentmetal-containing compound to be used by simple experiments carried outin the light of this disclosure. For example, when brines, such as arecommonly available in producing oil fields, are used in accordance withthe invention as the water in preparing gels, less of the startingpolyvalent metal-containing compound is required than when distilledwater is used. Stable gels have been prepared using brines having a widerange of dissolved solids content, e.g., greater than 60,000 ppm, andhigher, total dissolved solids, depending upon the particular polymerused having a degree of hydrolysis as defined above, and the brine used.Gelation rates are frequently faster when using said brines. Such oilfield brines commonly contain varying amounts of sodium chloride.calcium chloride, magne sium chloride. etc. Sodium chloride is usuallypresent in the greatest concentration.

When using polyaerylamides and related polymers having not more thanabout 14 percent of the carboxamide groups hydrolyzed to carboxylgroups, in accordance with the present invention, water having a totaldissolved solids content greater than 60.000 ppm by weight is apreferred medium for preparing the gels described herein. Good resultshave been obtained when using brines having a total dissolved solidscontent much greater than about 60,000 ppm by weight, e.g., up to atleast about 174,000 ppm by weight. Furthermore, of said total dissolvedsolids, the amount of polyvalent metal ions such as calcium, magnesium,etc., can

be greater than 6,000 ppm by weight. Good results have'been obtainedwhen using brines having greater than 12,000 ppm by weight of saidpolyvalent metal ions. As shown by the examples given hereinafter, theabove results are in marked contrast to the results obtained when thepolymers used have percent or more of the carboxamide groups hydrolyzedto carboxyl groups.

Another advantage is using strong field produced brines in preparingaqueous gels as described herein, in addition to the economic advantageof using readily available materials, is that the problem of disposingof such brines is lessened.

Suitable reducing agents which can be used in the practice of theinvention include sulfur-containing compounds such as sodium sulfite,sodium hydrosulfite, sodium metabisulfite, potassium sulfite, sodiumbisulfite, potassium metabisulfite, sodium sulfide, sodium thiosulfate,ferrous sulfate, thioacetamide, hydrogen sulfide, and others; andnonsulfur-containing compounds such as hydroquinone, ferrous chloride,phydrazinobenzoic acid, hydrazine phosphite, hydrazine dichloride, andothers. Some of the above reducing agents act more quickly than others,for example, sodium thiosulfate usually reacts slowly in the absence ofheat, e.g., heating to about l25130 F. The presently most preferredreducing agents are sodium hydrosulfite or potassium hydrosulfite.

The amount of reducing agent to be used in the practice of the inventionwill be a sensible amount, i.e., a small but finite amount which is morethan incidental impurities, but which is effective or sufficient toreduce at least a portion of the higher valence metal in the startingpolyvalent metal-containing compound to a lower polyvalent valencestate. Thus, the amount of reducing agent to be used depends, to someextent at least, upon the amount of the starting polyvalentmetalcontaining compound which is used. In many instances, it will bepreferred to use an excess of reducing agent to compensate for dissolvedoxygen in the water or brine, exposure to air during preparation of thegels, and possible contact with other oxidizing substances such as mightbe encountered in field operations. As a general guide, the amount ofreducing agent used will generally be within the range of from 0.1 to atleast 150, preferably at least about 200, weight percent of thestoichiometric amount required to reduce the metal in the startingpolyvalent to said lower polyvalent valence state, e.g., +6 Cr to +3 Cr.However, in some instances, it may be desirable to use amounts ofreducing agent outside said ranges. The use of such amounts is withinthe scope of the invention. Those skilled in the art can determine theamount of reducing agent to be used by simple experiments carried out inthe light of this disclosure.

Various methods can be used for preparing the aqueous gels used in thepractice of the invention. Either the metal-containing compound or thereducing agent can be first added to a solution or dispersion of thepolymer in water or brine, or said metal-containing compound and saidreducing agent can be added simultaneously to the solution or aqueousmedium containing the polymer. Generally speaking, where convenient, thepreferred method is to first disperse the polymer in the water or otheraqueous medium such as brine. The reducing agent is then added to thedispersion of polymer, with stirring. The metal-containingcompound isthen added to the solution or aqueous medium containing the polymer andthe reducing agent, with stirring. Gelation starts as soon as reductionof some of the higher valence metal in the starting polyvalentmetalcontaining compound to a lower valence state occurs. Thenewly-formed lower valence metal ions, for example +3 chromium obtainedfrom +6 chromium, effect rapid crosslinking of the polymer and gelationof the solution or aqueous medium containing same.

In another method, the aqueous gels can be formed in situ in theformation to be treated. This method is useful when the reducing agentis a reducing gas such as hydrogen sulfide or a hydrogen sulfidecontaining gas. The reducing gas can be naturally occurring in theformation or can be introduced into the formation. Thus, the polymer canbe dispersed in water and the metal-containing compound added to theresulting dis persion. Said dispersion is then pumped into contact withsaid formation. Upon contacting the reducing gas, either naturallyoccurring or injected following the injection of said dispersion, themetal in the metalcontaining compound will be reduced, e.g., Cr to Cr,and gelation will be effected. If desired, the gel can then be movedthrough said formation by the subsequent injection of a drive fluid,e.g., water.

It is also within the scope of the invention to prepare a dry mixture ofthe polymer, the metal-containing compound and the reducing agent, inproper proportions, and then add this dry mixture to the proper amountof water or brine.

An advantage of the invention is that ordinary ambient temperatures andother conditions can be used in practically all instances in preparingthe aqueous gels used in the practice of the invention or aqueousmediums containing same. However, in some instances, a small amount ofheat may be desirable to aid in the formation of the gel, e.g., heatingto a temperature of about l25l30 F.

Aqueous gels used in the practice of the invention can be preparedhaving a wide range of viscosities or firmness ranging from lowviscosity or highly mobile gels having a relatively low viscosity up tothick, viscous, somewhat elastic gels which are relatively nonmobile.The choice of gel viscosity or concentration will depend upon the use tobe made of the gel. For example, when the gel is to be used in a fluiddrive operation for the secondary recovery of oil, or otherwise injected into the pores of a nonfractured porous media, the gel viscosityor concentration can have any value which will permit the gel to beinjected into said pores for the intended purpose. The actual viscosityand/or gel strength of the gel will depend upon the type andconcentration of the polymer, the type and amount of starting polyvalentmetal compound used, and the type and amount of reducing agent used.

One presently preferred procedure is to prepare a relativelyconcentrated or high viscosity gel and dilute same to a viscosity orconcentration suited for the actual use of the gel. In many instances,this procedure results in a more stable gel, in addition to theadvantages mentioned above.

When employing said dilution technique a starting solution or dispersionof polymer containing, for example, 1,000 to 10,000 ppm (0.1 to 1 wt. ormore of polymer can be used. This solution or dispersion is then gelledby the addition of suitable amounts of polyvalent metal compound andreducing agent. After gelation has proceeded to the desired extent, theresulting gel can be diluted with water to the concentration orviscosity most suited for its intended use. For example, if the gel isto be used in a waterflood operation, it could be diluted to a nominal4,000, 2,500, 1,000, 500, 250, or less, ppm gel by the addition of asuitable amount of water. The more concentrated polymer solutions ordispersions usually have a faster rate of gelation. Thus, in mostinstances, it will be preferred to carry out the dilution soon after thecomponents of the gel have been added to the water or brine, e.g.,within about to 30 minutes. Preferably, the concentration of the polymerin the concentrated gel" will be at least twice that in the final gel.Dilution of the gel retards the rate of gelation. Thus, this dilutiontechniquecan be employed to control the gelation rate, if desired. Inmany instances, gels prepared by employing said dilution technique aremore stable. Another advantage of said dilution technique is that it isusually more convenient to weigh out and handle the larger quantities ofreagents.

We are aware that chromium ions having a valence of +3 have been used toreact with water-dispersible polymers such as polyacrylamides andpolysaccharides. See, for example, U.S. Pat. No. 3,114,651 to Gentileand US. Pat. No. 3,383,307 to Goetz. In such processes the chromiumcompound is added in a form wherein the chromium has an initial valenceof +3, e.g., CrCl;,, Cr(NO etc. In the practice of the presentinvention, the Cr ions must be newly formed, e.g., nascent ions formedin situ in the solution to be gelled by the reduction of Cr ions to Crions. We have found that aqueous gels of polymer solutions preparedusing such newly formed Cr ions have much better long term stabilitythan do gels prepared by the direct addition of Cr ions.

Gel instability is evidenced by precipitation and/or syneresis (bleedingor water separation). A severe test of gel stability is to prepare thegel and merely allow it to stand. We have found that gels which arestable for as long as 48 hours are usually stable for. a month orlonger. We have also found that formation solids such as sandstone andlimestone improve gel stability.

Generally speaking, the pH of the final solution of the gelling reagentsis preferably less than 7, more preferably in the order of 6. Ingeneral, pH is not controlling, but higher pH values retard gelationrate. In general, the pH of the gelling solution will depend upon the reducing agent used. If'desired, the pH can be adjusted by the addition ofa suitable acid, depending upon the reducing agent used.

Herein and in the claims, unless otherwise specified, the aqueous gelsused in the practice of the invention are defined for convenience, andnot by way of limitation, in terms of the amount of polymer containedtherein, irrespective of whether or not all the polymer has entered intothe gelforming reaction. For example, a 1 weight percent or 10,000 ppmgel is a gel which was prepared from a starting polymer solution ordispersion which contained 1 weight percent or 10,000 ppm by weight ofpolymer. The same system is employed for the gels prepared by theabove-described dilution technique.

As indicated above, the above-described aqueous gels are particularlyuseful in fluid drive operations for the secondary recovery of oil.Saidgels are applicable for decreasing the mobility of a drive fluid,such as water or other fluids, or decreasing the permeability ofnonfractured porous formations prior to or during secondary recoveryoperations, such as fluid drive processes, and also for water shutofftreatments in producing wells. In such processes the aqueous gels can bein- 5 jected into the formation prior to or subsequent to anotherinjected fluid. For example, in one particular useful application, aslug of aqueous gel can be injected after a previously injected slug ofa fluid such as a detergent and/or oil-containing fluid which serves toloosen the oil from the formation. Said slug of gel can then be followedby water to push both of said slugs toward the production well. In oneembodiment of the invention, a conventional waterflood or gas drive iscarried out in conventional manner until the drive fluid breaks throughinto the production well in excessive amounts. An above-described gel isthen pumped down the well and into the nonfractured porous formation inany suitable manner, any suitable amount, and for any desired period oftime sufficient to obtain the desired in-depth penetration and decreasein mobility of the drive fluid, or decrease in permeability of the highpermeability zones of said formation. Usually, an in-depth penetrationof from 10 to 1,000, preferably to 900, feet from the injection wellwill be sufficient. However, this can vary from formation to formationand penetrations outside said ranges can be used. For example, there canbe injected into the formation via the injec tion well from about 0.001to about 0.5 pore volume of a gel in accordance with the invention overa suitable period of time ranging from one day to six months. Or, theinjection of the gel can be carried out by injecting a slug of about 200to 5,000 barrels of gel into the well and then into the formation.Injection in one of the above manners will provide a flood frontadjacent the oil to be produced. If desired, an ordinary brine or watercan then be employed to drive this slug or band or front of'gel onthrough the formation to the production well. If desired, in order toavoid any sharp demarcations in viscosity or mobility of the gel, whichcould adversely affect the relative mobility of the flood me dium andthe oil, and cause channeling, the viscosity of concentration of the gelcan gradually be lessened through a series of incremental decreasesrather than discontinuing the injection thereof abruptly. Also, ifdesired, said slug of brine or water can be followed with another slugof gel.

In another embodiment of the invention, the nonfractured porousformation can be treated prior to carrying out the fluid drive secondaryrecovery operations. This embodiment is particularly applicable wherethere is a good knowledge of the nature of the formation. Thus, in sucha formation where the oil-bearing strata are interspersed with morepermeable porous strats which contain no oil, or an insufficient amountof oil to make secondary recovery operations economical, but which morepermeable strata would still act as a thief zone, the formation can betreated in accordance with the invention prior to initiating the fluiddrive operation.

In still another embodiment, the invention can be applied .to producingwells, either oil wells or gas wells, where there is a more porousnonhydrocarbon-bearing strata adjacent the hydrocarbon-bearing strata.For example, such a condition can exist where there is a water sandadjacent the hydrocarbon-bearing sand and the water intrudes into theborehole and interferes with the production of hydrocarbon. In suchinstances, the formation can be treated in accordance with the inventionto shut off the flowof water. The method of carrying out such a watershutoff treatment is substantially the same as described above inconnection with fluid drive operations.

In any of the above-described embodiments of the invention, a slug ofungellcd polymer can be injected ahead of the aqueous gel. The ungelledpolymer can thus be used to satisfy the absorption requirements of theformation, resulting in less absorption and more efficient utilizationof the aqueous gel. The initial injection of ungelled polymer also aidsin reducing face plugging where this is a problem.

It is also within the scope of the invention to carry out the gelinjection techniques of the invention periodically or intermittently, asneeded, during the course of a fluid drive secondary operation, orduring the production of oil from a producing well.

1n all of the above operations, the injection of the gel can be carriedout in conventional manner. If desired, a gel of suitable viscosity orconcentration can be injected as the drive fluid per se. Gels injectedin accordance with the invention can be prepared in advance, stored insuitable tanks, and then pumped into the well. Or, said gels can beformed in a conduit leading to the injection well, Orin the tubing inthe well itself, and

then injected into the formation. Thus, the required amounts of polymer,polyvalent metal compound, and reducing agent can be metered into thetubing in the well, mixed therein, and then injected into the formation.If desired, selected portions of the formation can be isolatedmechanically, as by the use of packers, and other means known to theart. for treatment in accordance with the invention.

The above-described aqueous gels can comprise, or can be employed as,drilling fluids in the drilling of wells in any manner known to the artfor the use of drilling fluids. Such gels can be employed without theaddition of other materials thereto. However, if desired, weightingagents such as barium carbonate, barium sulfate, amorphous silica, etc.,can be included in the drilling fluids comprising said aqueous gels. Ifdesired,

other constituents present under the particular well conditionsexisting. As indicated, in selecting such additives for use in aparticular drilling fluid, care should be taken to avoid materials whichare not compatible with the aqueous gels. The use of such additives willbe governed in part by the viscosity and fluid loss properties desiredin the drilling fluid. Thus, is the situation in connection withconventional drilling fluids, pilot tests should be run to determine theproperties desired for the aqueous gel used as the drilling fluid, or anaqueous gel containing one of the above-described additives, todetermine the optimum results or properties desired for the drillingfluid under the particular well conditions existing.

The following examples will serve to further illustrate the invention.

EXAMPLE 1 A series of aqueous gels was prepared using variouscommercially available polyacrylamides having different molecularweights and different amounts of the carboxamide groups thereinhydrolyzed to carboxylic groups. Said gels were prepared using a typicalsample of brine produced in the East Hull Silk Field in Archer County,Texas. This brine analyzed as follows:

ppm by weight NaCl 124.000 Calll 34,200 M gCl ZH O 16,000 174,200

Each of the aqueous gels tested contained 2,000 ppm by weight of polymerand was prepared as follows. Two grams of each polymer were added to 1liter of said brine. To each of the resulting solutions there was added,with stirring, an amount of a 10 weight percent solution of sodiumhydrosulfite sufficient to provide 300 ppm by weight of Na S O,,. Therewas then added, with stirring, an amount of a 10 weight percent solutionof sodium dichromate sufficient to provide 300 ppm of Na Cr O '2H O.Properties of each of the resulting gels, or solutions, are set forth inTable 1 below.

TABLE 1 Polymer Solution Viscosity/ Molecular Percent Viscosity at 72hrs. Polymer Weight Hydrolyzed cp. cp. Remarks PF 1 160 15 X l 3.6 5.4l()0,000 clear solution without precipitate;

excellent gel PF 1 l 8 X 10 1 1 1 1.5 100,0()0 clear solution; gel notas smooth as PF 1 160 gel WC 500 3.5 X 10" 4.4 2.4 clear solution, butdid not form gel DP 1000 10 X 10 21 5.9 2.3 solution precipitated;gelled, hut

gel broke in 24 hrs. PF I 120 16 x 10 25 6.3 2.1 solution turbid;gelled. but gel broke in 24 hrs. WC 773 16 X 10 3.3 25 clear solution,but did not form gel PF 1 130 10.5 X 11) 2.7 solution badlyprecipitated; did not form gel "'Measured on Broukl'lekl "*"Muasurctl onBrookliekl viscometer at 6 rpm.

other additives compatible with the aqueous gels can also be included inthe drilling fluid. Thus, the drilling fluids can include clays such asbentonite, attapulgus clay, fluid loss agents, etc. 1t should beunderstood that not all of these additives or constituents willnecessarily be present in any one drilling fluid and that the amount ofany particular additive used will be governed by the "scnmeter at 6 rpm,after solution hydrated 24 hrs.

solved solids. The results obtained were essentially the same as setforth in Table l.

The above data show that polyacrylamides wherein 15 percent of thecarboxamide groups have been hydrolyzed to carhoxylic groups cannot beused in preparing stable gels of polyacrylamide solutions in strongbrines. Said data also show that polyacrylamides wherein 11 percent ofthe carboxamide groups have been hydrolyzed can be used in preparingstable gels. With brines containing less dissolved solids than the brineused in this Example I, e.g., at least about 40,000 ppm by Weight,polymers having a degree of hydrolysis of more than 1 1 percent, e.g.,up to about 14 percent, can be used in the practice of the invention.

EXAMPLE [I An aqueous gel was prepared from a solution which initiallycontained 1000 parts per million (0.1 weight percent) of a substantiallylinear polyacrylamide having a molecular weight of about 16 X and adegree of hydrolysis of about 25 percent. Said gel was prepared as givenbelow.

One gram of said polyacrylamide was added to 1 liter of synthetic EastHull Silk Brine to give an ungelled solution of polyacrylamide. Saidsynthetic brine was pre pared by adding to distilled water the requiredamounts of sodium chloride, calcium chloride, and magnesium chloridenecessary to approximate the concentration of cations found in a typicalsample of actual field produced brine. Said synthetic brine had thefollowing composition:

Na 48,900 ppm Ca 12,300 ppm Mg 1,900 ppm Cl 104,000 ppm To a portion ofsaid ungelled solution of polyacrylamide there was added, with stirring,suffieient sodium hydrosulfite (10% solution in distilled water) to give300 ppm by weight of Na S O To the resulting solution there was thenadded, with mixing, sufficient sodium dichromate (10% solution indistilled water) to give 300 ppm by weight of Na Cr O -2H O. The resulting gel was a nominal 1,000 ppm gel (01 weight percent).

Said 1,000 ppm gel and said solution of ungelled polyacrylamide werethen used to carry out water diversion tests in a linear displacementmodel (sand pack) prepared as follows. A Lucite pipe 12 inches longhaving an internal diameter of 1.0625 inches was filled with Ottawasand. The filled pipe (pack) was then evacuated to remove air. Theevacuated pack was then flooded with CO gas to further remove any air.The pack was then again evacuated. The pack was then flooded with waterfrom the bottom so as to remove all traces of gas therefrom. The columnwas mounted in a horizontal position then flooded with an oil having aviscosity of 50 centipoises at 75 F. so as to establish an oilsaturation and residual connate water saturation condition. The pack wasthen waterflooded with the above-described synthetic East Hull Silkbrine for a sufficient period of time to give an exit water-to-oil ratioin excess of 100:1, and thus insure that all mobile oil had been removedtherefrom. The pack was then considered to be in a flooded-out statewith residual oil saturation.

The thusprepared linear displacement model had a pore volume of 65milliliters, a porosity of 36 percent, and a residual oil saturation(Sor) of 0.15. The pack was mounted in a horizontal position andprovided with a pressure tap at 3 inches from the inlet end. Waterdiversion tests were then carried out as described below, with allliquids being pumped into said inlet end of the pack.

In making water diversion tests using the abovedescribed pack, thevarious liquids are pumped into the pack at uniform rates simulating alinear velocity of about feet per day. Pressure readings are taken atthe inlet end and at the pressure tap located three inches from theinlet. The data thus obtained are employed to calculate mobility valuesusing Darcys linear flow equation. Said equation can be written asfollows:

wherein the factor (K/p.) represents mobility in millidarcys divided bythe viscosity of the liquid, Q represents flow rate in cubic centimetersper second, L represents the length of the pack in centimeters, Arepresents cross-sectional area of the pack in square centimeters, andAP represents the differential pressure in atmospheres. Since L and Aare constant for any given model, mobility can be readily calculatedfrom Q/AP. If desired, in employing the data, mobility values can beplotted as the ordinate versus cumulative volume injected as theabscissa.

The test was carried out by first introducing 50 m1 of oil having aviscosity of 50 cp at F. to establish the mobility of the pack to oil.Mobility measurements were then carried out by first introducing a totalof 750 milliliters of said brine at a uniform rate such that the linearvelocity through the pack was about 70 feet per day. After 500milliliters of said brine had passed through the pack, it was determinedthat the water-tooil ratio in the pack effluent was greater than :1,showing that all the mobile oil had been removed from the pack. Next, atotal of 250 milliliters of the abovedescribed ungelled solution ofpolyacrylamide containing 1,000 ppm (0.1 weight percent) of polymer wasintroduced at the same rate. This was followed by the injection ofanother 250 milliliters of said brine at the same rate. Next,milliliters of the above-described gelled solution of polyacrylamidecontaining 1,000 ppm (0.1 weight percent) of polymer was introduced atthe same rate. The pack was shut in for two days to allow the gel toset. This was followed by 65 milliliters of said brine (1 pore volume)at the same rate to remove any displaceable gel. This was followed with1713 milliliters of the 50 cp oil at the same rate to determine theeffect the gelled solution had on the packs mobility to oil. This wasfollowed by 1876 milliliters of said brine at the same rate to determinethe residual mobility change effected by the gel. Throughout theintroduction of said liquids, pressure readings were taken at theentrance to the pack, and a point 3 inches from the entrance. Mobilityvalues, K/[.L, for each liquid were calculated from said injection rateand the pressure readings, described above. Results for the secondsection of the pack, i.e.. between the pressure tap and the outlet ofthe pack, are set forth in Table 11 below.

TABLE ll Cumulative Slug Volume Total Cumulative Mobility 01 LiquidInjected, Vol. 01' Liquid Injected. (K/l -l Liquid Injected ml. ml.md/cp RRF Oil 60 60 350 120 120 350 150 150 350 1.0 Brine 610 760 75006X7 837 7500 750 900 7600 1.0 Ungellcd Solution of Polymer 85 985 i 3700116 1016 3400 250 1150 3400 2. Brim: 130 1280 7500 165 1315 7400 2501400 7500 1.0 Gelled Solution of Polymer 74 1474 4600 90 1490 4700 1201520 4300 1.8 Pack shut in two days after gel injection. Pack flushedwith one pore volume of brine (65 ml.). Oil 138 1723 360 632 2217 360882 2467 350 1713 3298 340 1.0 Brine 496 3794 3500 1 147 4445 4000 16364934 3900 1876 5174 3800 2.0

Referring to Table II, the mobility of the oil in the pack initially was350 millidarcys per centipoise. The mobility of the brine was 7600millidarcys, per cp. The mobility of the ungellcd solution ofpolyacrylamide at the end of the injection thereof was 3,400 millidarcysper centipoisc. The mobility of the second brine injection increment was7,500, showing that the ungellcd solution of polyacrylamide was elutedor washed out the pack, and that said ungellcd solution was noteffective in reducing the permeability of the column and/or decreasingthe mobility of the brine. The mobility of the gelled solution ofpolyacrylamide was 4,300 millidarcys per centipoise. This indicated thatthe gelled solution of polyacrylamide was not superior to the ungelledsolution of polyacrylamide for increasing resistance factors. Themobility of the final slug of oil injected stabilized at a value in theorder of 350 millidarcys per centipoise, showing that both the gelledand the ungellcd solutions of polyacrylamide had not materially affectedthe permeability of the pack to the oil. The mobility of the final slugof brine was in the order of 3,800 md per cp, indicating a residualresistance factor of 2 to thebrine. From the above data, it is evidentthat the gelled solution of polyacrylamide used in this example did notsignificantly reduce the mobility of the brine in said sand pack. Thus,one would conclude that this gelled solution of polyacrylamide would notbe effective in reducing the permeability of the more porous sections ofa formation and thus divert flood water to the less porous sections of aformation.

EXAMPLE 111 An aqueous gel was prepared from a solution which initiallycontained 1,000 parts per million (0.1 weight percent) of asubstantially linear polyacrylamide having a molecular weight of about X10 and a degree of hydrolysis of about 3.6 percent. Said gel wasprepared as follows. One gram of said polyacrylamide was added to 1liter of the above-described synthetic East Hull Silk brine. To aportion of this ungelled solution there was then added, with mixing,sufficient sodium hydrosulfite (in 10% solution) to give 03 gram perliter of Na S O To the resulting solution there was then added, withmixing, sufficient sodium dichromate (in 10% solution) to give 0.3 gramper litter of Na Cr- O '2- H O. The resulting gel was a nominal 1,000ppm gel (0.1 weight percent).

Said 1,000 ppm gel and said solution of ungelled polyacrylamide werethen used to carry out water diversion tests in a linear displacementmodel (sand pack) prepared essentially as described above in Example 11.The linear displacement model used had a pore volume of milliliters, aporosity of 36 percent, and a residual oil saturation (Sor) of 0.15.

Mobility measurements were carried out by first introducing a total of100 ml of oil having a viscosity of 50 cp at F. to establish mobility ofthe pack to oil. There was then introduced a total of 1,460 millilitersof said synthetic brine at a rate such that the velocity through thecolumn was about 70 feet per day. After 1,000 milliliters of said brinehad passed through the pack, it was determined that the water-to-oilratio in the pack effluent was greater than :1, showing that essentiallyall the mobile oil had been removed from the pack. Next, a total of 314milliliters of said ungelled solution of polyacrylamide was introducedat the same rate. This was followed by 386 milliliters of said brine atthe same rate. Next, ml of said gelled solution of polyacrylamide wereinjected at the same rate. The pack was shut in for two days to permitthe gel to set. This was followed by 84 ml of said brine 1.3 porevolume) to displace any movable gel. Next. 1,502 ml of said 50 ep oilwas injected at the same rate to deter mine the pack mobility to oilafter said gel injection. Next, 1.038 ml of said brine was injected todetermine final resistance effects of the gel. Throughout theintroduction of said liquids, pressure readings were taken at theentrance to the pack and a point 3 inches from the entrance to thepackfMobility, K/y. of each liquid, was calculated from said injectionrateand said pressure readings, as described above in Example 11. Theresults for the second section of the pack, i.e., between the pressuretap and the outlet of the pack. are set forth in Table III below.

While certain embodiments of the invention have been described forillustrative purposes, the invention TABLE III Cumulative Slug VolumeTotal Cumulative Mobility of Liquid Injected, Vol. of Liquid Injected,(K/p.) Liquid Injected ml, ml. md/cp RRF Oil 40 40 280 60 60 280 I I00280 I .0 Brine I I30 I230 6000 I395 I495 5600 I460 I560 5700 1.0Ungelled Solution of Polymer I I4 I674 2400 164 I724 2300 314 I874 24002.4 Brine 25 I 2 I 25 5400 326 2200 5200 386 2260 5300 L1 GelledSolution of Polymer 70 2330 2500 90 2350 650 I 2380 680 I35 2395 500 1L4 Pack shut in two days after gel injection Pack flushed with 1.3 porevolumes of brine (84 ml.). Oil 3331 48 I042 3521 52 I252 373] 52 I502398] 54 5.2 Brine 204 4185 I8 376 4357 I9 620 4601 I9 I038 50]) I8 3l7.0

The data in Table III shows that the initial mobility of the pack to oilwas 280 md/cp, and the mobility of the pack to brine was 5,700 md/cp atresidual oil saturation. During the injection of the ungelled solutionof polyacrylamide, the mobility decreased to 2,400 md/cp. However, uponthe subsequent injection of brine, the mobility increased to 5,300md/cp, indicating that said ungelled solution was ineffective as amobility control agent. Upon injection of the gelled solution ofpolyacrylamide, the mobility decreased to 500 md/cp. After allowing thegel to set two days, 1.3 pore volumes of brine were injected to removeany displaceable gel. Upon injection of oil to determine the mobility ofthe treated pack to oil, it was found that the mobility to oil had beenreduced to about 50 md/cp, showing that the gelled solution ofpolyacrylamide had reduced the mobility of the pack to oil. The finalbrine injection gave a mobility of 18 md/cp, showing that the gelledsolution of polyacrylamide was extremely effective in reducing thepermeability of the pack to brine.

The above data show that the gelled solution of polyacrylamide gave aresidual resistance factor (RRF) of 5.2 with respect to the oil, and anRRF of 317 with respect to the brine. This shows that the gelledsolution of polyacrylamide was much more effective in reducing thepermeability to brine than in reducing the permeability to oil.

The polyacrylamides used in Examples II and III had substantially thesame properties with the exception that the polyacrylamide of Example IIwas about percent hydrolyzed whereas the polyacrylamide of EX- ample IIIwas only about 3.6 percent hydrolyzed. Comparing the results of ExamplesII and III shows that the gelled solution of polyacrylamide in ExampleII was not effective in reducing the mobility of brine in the pack. Incontrast, the gelled solution of polyacrylamide in Example III was veryeffective in reducing the mobility of the brine.

is not limited thereto. Various other modifications or embodiments ofthe invention will be apparent to those skilled in the art in view ofthis disclosure. Such modifi' cations or embodiments are within thespirit and scope of the disclosure.

We claim:

1. An aqueous medium, comprising water to which there has been added:

a water-thickening amount ofa water-dispersible polymer selected fromthe group consisting of: polyacrylamides and polymethacrylamides whereinfrom 0.1 to about 14 percent of the carboxamide groups are hydrolyzed tocarboxyl groups; crosslinked polyacrylamides and crosslinked polymetha'crylamides wherein from 0.1 to about 14 percent of the carboxamidegroups are hydrolyzed to carboxyl groups; copolymers of acrylamide withanother ethylenically unsaturated monomer copolymerizable therewith,sufficient acrylamide being present in the monomer mixture to impartsaid water-dispersible properties to the resulting copolymer when it ismixed with water, and wherein from 0.1 to about l4 percent of thecarboxamide groups are hydrolyzed to carboxyl groups; and mixtures ofsaid polymers;

an amount of a water-soluble compound of a polyvalent metal wherein thevalence state of the metal therein is capable of being reduced to alower polyvalcnt valence state and which is sufficient to supply atleast about 3 X 10 gram atoms of said polyvalent metal per gram of saidpolymer; and

an amount of a water-soluble reducing agent which is effective to reduceat least a portion of said metal to said lower valence state;

the initial total dissolved solids content of said firstmentioned waterbeing at least about 40,000 ppm by weight.

2. An aqueous medium according to claim I wherein there has been addedto said first-mentioned water:

from 0.0025 to weight percent of said polymer.

based upon the weight of said water;

from 0.05 to 60 weight percent of said polyvalcnt metal compound basedupon the weight of said polymer; and

from 0.1 to at least about 200 percent of the stoichiometric amount ofsaid reducing agent required to reduce said polyvalcnt metal to saidlower polyvalent valence state.

3. An aqueous medium according to claim 1 wherein said polymer is asubstantially linear polymer of acrylamide.

4. An aqueous medium according to claim 3 wherein there has been addedto said first-mentioned water:

from 0.01 to 1.5 weight percent of said polymer,

based upon the weight of said water;

from 0.5 to weight percent of said polyvalcnt metal compound. based uponthe weight of said polymer; and

from Oil to at least about 150 weight percent of the stoichiometricamount of said reducing agent re- I quired to reduce said polyvalcntmetal to said lower polyvalcnt valence state.

5. An aqueous medium according to claim '4 wherein:

said compound of a polyvalcnt metal is a compound ofchromium wherein thevalence of the chromium is +6 and the valence of at least a portion ofsaid chromium is reduced to +3.

6. An aqueous medium according to claim 5 wherein:

said polyvalcnt metal compound is selected from the group consisting ofsodium dichromate. potassium dichromate, and mixtures thereof: and

said reducing agent is selected from the group consisting of sodiumhydrosulfite. potassium hydrosulfite. sodium thiosulfate, sodiummetabisulfitc. potassium metabisulfitc. and mixtures thereof.

7. An aqueous medium according to claim 1 wherein:

said polymer is a substantially linear'polymer of acrylamide whereinfrom 0.] to about 12 percent of the carhoxamide groups therein arehydrolyzed to carboxyl groups; and

the' initial total dissolved solids content of said firstmentioned wateris greater than about 60.000 ppm by weight.

8. An aqueous medium according to claim 7 wherein there has been addedto said first-mentioned water:

from 0401 to L5 weight percent of said polymer.

based upon the weight of said water;

from 0.5 to 30 weight percent of said polyvalcnt metal compound. basedupon the weight of said polymer: and

from 0.1 to at least about l50 weight percent of the stoichiometricamount of said reducng agent required to reduce said polyvalcnt metal tosaid lower polyvalcnt valence state.

9. An aqueous medium according to claim 8 wherein:

said reducing agent is selected from the group consisting ofhydroquinone. sodium sulfide. hydrogen i said compound of a polyvalcntmetal is a compound of chromium wherein the valence of the chromium is+6 and the valence of at least a portion of said chromium is reduccd to+3.

10. An aqueous medium according to claim 9 wherein:

said polyvalcnt metal compound is sodium dichromate; and i said reducingagent is sodium hydrosulfite. 11. A method for producing an aqueous gel.which method comprises:

thickening a brine having an initial total dissolved solids content ofat least about 40,000 ppm by weight by adding thereto a thickeningamount of a water-dispersible polymer selected from the group consistingof: polyacrylamides and polymethacrylamides wherein from 0.1 to about 14percent of the carboxamide groups are hydrolyzed to carboxyl groups;crosslinked polyacrylamides and crosslinked polymethacrylamides whereinfrom 0.1 to about 14 percent of the carboxamide groups are hydrolyzed tocarboxyl groups; copolymers of acrylamide with another ethylcnicallyunsaturated monomer copolymerizable therewith, sufficient acrylamidebeing present in the monomer mixture to impart said water-dispersibleproperties to the resulting copolymer when it is mixed with water. andwherein from 0.] to about 14 percent of the car boxamidc groups arehydrolyzed to earboxyl groups; and mixtures of said polymers; and

causing gelation of the so-thickened brine by adding thereto anamountofa water-soluble compound of a'polyvalent metal wherein thevalence state of the metal therein is capable of being reduced to alower polyvalcnt valence state and which is sufficient to supply atleast about 3 X 10" gram atoms of said polyvalcnt metal per gram of saidpolymer, and an amount of a water-soluble reducing agent which iseffective to reduce at least a portion of said metal to said lowervalence state. 12. A method according to claim 11 wherein: said brine isthickened by adding thereto from 0.0025 -to 5 weight percent of saidpolymer; based on the weight of the brine: and 1 gelation of saidthickened brine is caused by adding thereto from 0.05 to weight percentof said polyvalcnt metal compound based on the weight of said polymer.and from 0.1 to at least about 200 percent of the stoichiometric amountof said reducing agent required to reduce said polyvalcnt metal to'saidlower valence state.

[3. A method according to claim 12 wherein said polyvalcnt metalcompound is a compound of chromium wherein the valence of the chromiumis +6 and the valence of at least a port-ion ofsaid chromium is reducedto +3.

14. A method according to claim 13 wherein:

said chromium compound is selected from the group consisting of ammoniumchromate. ammonium dichromate. the alkali metal chromates anddichromates. chromium trioxide. and mixtures thereof.

15. A method according to claim 14 wherein said polymer is asubstantially linear polymer of acrylamidc.

16. A method according to claim 15 wherein:

said reducing agentis selected from the group consisting ofhydroquinone. sodium sulfide. hydrogen sulfide, sodium hydrosulfite.sodium metabisulfite.

potassium sulfite, sodium bisulfite, potassium metabisulfite, sodiumsulfite, sodium thiosulfate, ferrous sulfate. ferrous chloride,phydrazinobenzoic acid, hydrazine phosphitc, hydrazine dihydrochloride,and mixtures thereof.

17. A method according to claim 12 wherein:

said polymer is a substantially linear polymer of acrylamide:

said polyvalent metal compound is sodium dichromate; and

said reducing agent is sodium hydrosulfitc.

18. A method according to claim 11 wherein:

said polymer is a substantially linear polyacrylamide wherein from 0.1to about 12 percent of the carboxamidc groups therein are hydrolyzed tocar boxy] groups; and

the initial total dissolved solids content of said brine is greater thanabout 60,000 ppm by weight.

19. A method according to claim 18 wherein:

said brine is thickened by adding thereto from 0.01 to 1.5 weightpercent of said polymer, based on the weight of said brine; and

gelation of said thickened brine is caused by adding thereto from 0.5 toweight percent of said polyvalcnt metal compound based on the weight ofsaid polymer, and from 0.1 to at least about weight percent of thestoichiomctric amount of said reduc ing agent required to reduce saidpolyvalent metal to said lower valence statev 20. A method according toclaim 19 wherein said polyvalent metal compound is a compound ofchromium wherein the valence of the chromium is +6 and the valence of atleast a portion of said chromium is re duced to +3.

21. A method according to claim 20 wherein: said reducing agent isselected from the group consisting of hydroquinone, sodium sulfide,hydrogen sulfide, sodium hydrosulfite, sodium metabisulfite. potassiumsulfite, sodium bisulfite, potassium metabisulfite, sodium sulfite,sodium thiosulfate, ferrous sulfate, ferrous chloride, phydrazinobenzoicacid, hydrazine phosphite, hydrazine dihydrochloride, and mixturesthereof. 22. A method according to claim 19 wherein: said polyvalentmetal compound is sodium dichromate; and said reducing agent is sodiumhydrosulfite.

1. AN AQUEOUS MEDIUM COMPRISING WATER TO WHICH THERE HAS BEEN ADDED: AWATER-THICKENING AMOUNT OF A WATER-DISPERSIBLE POLYMER SELECTED FROM THEGROUP CONSISTING OF: POLYACRYLAMIDES AND POLYMETHACRYLAMIDES WHEREINFROM 0.1 TO ABOUT 14 PERCENT OF THE CARBOXAMIDE GROUPS ARE HYDROLYZED TOCARBOXYL GROUPS CROSSLINKED POLYACRYLAMIDES AND CROSSLINKEDPOLYMETHACRYLAMIDES WHEREIN FROM 0.1 TO ABOUT 14 PERCENT OF THECARBOXAMIDE GROUPS ARE HUDROLYZED TO CARBOXYL GROUPS COPOLYMERS OFACRYLAMIDE WITH ANOTHER ETHYLENICALLY UNSATURATD MONOMER COPOLYMERIZABLETHEREWITH SUFFICIENT ACRYLAMIDE BEING PRESENT IN THE MONOMER MIXTURE TOIMPART SAID WATER-DISPERSIBLE PROPERTIES TO THE RESULTING COPOLYMER WHENIT IS MIXED WITH WATER AND WHEREIN FROM 0.1 TO ABOUT 14 PERCENT OF THECARBOXAMIDE GROUPS ARE HYDROLYZED TO CARBOXYL GROUPS, AND MIXTURES OFSAID POLYMERS, AN AMOUNT OF WATER-SOLUBLE COMPOUND OF A POLYVALENT METALWHEREIN THE VALENCE STATE OF THE METAL THEREIN IS CAPABLE OF BEINGREDUCED TO A LOWER POLYVALENT VALENCE STATE AND WHICH IS SUFFICIENT TOSUPPLY AT LEAST 3X 10-6 GRAM ATOMS OF SAID POLYVALENT METAL PER GRAM OFSAID POLYMER AND AN AMOUNT OF A WATER-SOLUBLE REDUCING AGENT WHICH ISEFFECT TIVE TO REDUCE AT LEAST A PORTION OF SAID METAL TO SAID LOWERVALENCE STATE, THE INITIAL TOTAL ISSOLVED SOLIDS CONTANT OF SAIDFIRST-MENTIONED WATER BEING AT LAST ABOUT 40,000 PPM BY WEIGHT.
 2. Anaqueous medium according to claim 1 wherein there has been added to saidfirst-mentioned water: from 0.0025 to 5 weight percent of said polymer,based upon the weight of said water; from 0.05 to 60 weight percent ofsaid polyvalent metal compound based upon the weight of said polymer;and from 0.1 to at least about 200 percent of the stoichiometric amountof said reducing agent required to reduce said polyvalent metal to saidlower polyvalent valence state.
 3. An aqueous medium according to claim1 wherein said polymer is a substantially linear polymer of acrylamide.4. An aqueous medium according to claim 3 wherein there has been addedto said first-mentioned water: from 0.01 to 1.5 weight percent of saidpolymer, based upon the weight of said water; from 0.5 to 30 weightpercent of said polyvalent metal compound, based upon the weight of saidpolymer; and from 0.1 to at least about 150 weight percent of thestoichiometric amount of said reducing agent required to reduce saidpolyvalent metal to said lower polyvalent valence state.
 5. An aqueousmedium according to claim 4 wherein: said compound of a polyvalent metalis a compound of chromium wherein the valence of the chromium is +6 andthe valence of at least a portion of said chromium is reduced to +3. 6.An aqueous medium according to claim 5 wherein: said polyvalent metalcompound is selected from the group consisting of sodium dichromate,potassium dichromate, and mixtures thereof; and said reducing agent isselected from the group consisting of sodium hydrosulfite, potassiumhydrosulfite, sodium thiosulfate, sodium metabisulfite, potassiummetabisulfite, and mixtures thereof.
 7. An aqueous medium according toclaim 1 wherein: said polymer is a substantially linear polymer ofacrylamide wherein from 0.1 to about 12 percent of the carboxamidegroups therein are hydrolyzed to carboxyl groups; and the initial totaldissolved solids content of said first-mentioned water is greater thanabout 60,000 ppm by weight.
 8. An aqueous medium according to claim 7wherein there has been added to said first-mentioned water: from 0.01 to1.5 weight percent of said polymer, based upon the weight of said water;from 0.5 to 30 weight percent of said polyvalent metal compound, basedupon the weight of said polymer; and from 0.1 to at least about 150weight percent of the stoichiometric amount of sAid reducng agentrequired to reduce said polyvalent metal to said lower polyvalentvalence state.
 9. An aqueous medium according to claim 8 wherein: saidreducing agent is selected from the group consisting of hydroquinone,sodium sulfide, hydrogen sulfide, sodium hydrosulfite, sodiummetabisulfite, potassium sulfite, sodium bisulfite, potassiummetabisulfite, sodium sulfite, sodium thiosulfate, ferrous sulfate,ferrous chloride, p-hydrazinobenzoic acid, hydrazine phosphite,hydrazine dihydrochloride, and mixtures thereof; and said compound of apolyvalent metal is a compound of chromium wherein the valence of thechromium is +6 and the valence of at least a portion of said chromium isreduced to +3.
 10. An aqueous medium according to claim 9 wherein: saidpolyvalent metal compound is sodium dichromate; and said reducing agentis sodium hydrosulfite.
 11. A METHOD FOR PORODUCING AN AQUEOUS GEL WHICHMETHOD COMPRISES: THICKENING A BRINE HAVING AN INITIAL TOTAL DISSOLVEDSOLIDS CONTENT OF AT LEAST ABOUT 40,000 PPM BY WEIGHT BY ADDING THERETOA THICKENING AMOUNT OF A WATER-DISPERSIBLE POLYMER SELECTED FROM THEGROUP CONSISTING OF: POLYACRYLAMIDES AND POLYMETHACRYLAMIDES WHEREINFROM 0.1 TO ABOUT 14 PERCENT OF THE CARBOXAMIDE GROUPS ARE HYDROLYZED TOCARBOXYL GROUPS CROSSLINKED POLYACRYLAMIDES AND CROSSLINKEDPOLYMETHACRYLAMIDES WHEREIN FROM 0.1 TO ABOUT 14 PERCENT OF THECARBOXAMIDE GROUPS ARE HYDROLYZED TO CARBOXYL GROUPS COPOLYMERS OFACRYLAMIDE WITH ANOTHER ETHYLENICALLY UNSATURATED MONOMERCOPOLYMERIZABLE THEREWITH SUFFICIENT ACRYLAMIDE BEING PRESENT IN THEMONOMER MIXTURE TO IMPART SAID WATER-DISPERSIBLE PROPERTIES TO THERESULTING COPOLYMER WHEN IT IS MIXED WITH WATER AND WHEREIN FROM 0.1 TOABOUT 14 PERCENT OF THE CARBOXAMIDE GROUPS ARE HYDROLYZED TO CARBOXYLGROUPS, AND MIXTURES OF SAID POLYMERS AND CAUSING GELATION OF THESO-THICKENED BRINE BY ADDING THERETO AN AMOUNT OF A WATER-SOLUBLECOMPOUND OF A POLYVALENT METAL WHEREIN THE VALENCE TATE OF HE METALTHEREIN IS CAPABLE OF BEING REDUCED TO A LOWER POLYVALENT VALENCE STATEAND WHICH IS SUFFICIENT TO SUPPLY AT LEAST ABOUT 3 X 10-6 GRAM ATOMS OFSAID POLYVALENT METAL PER GRAM OF SAID POLYMER AND AN AMOUNT OF AWATER-SOLUBLE REDUCING AGENT WHICH IS EFFECTIVE TO EDUCE AT LEAST APORTION OF SAID METAL TO SAID LOWER VALENCE STATE.
 12. A methodaccording to claim 11 wherein: said brine is thickened by adding theretofrom 0.0025 to 5 weight percent of said polymer, based on the weight ofthe brine; and gelation of said thickened brine is caused by addingthereto from 0.05 to 60 weight percent of said polyvalent metal compoundbased on the weight of said polymer, and from 0.1 to at least about 200percent of the stoichiometric amount of said reducing agent required toreduce said polyvalent metal to said lower valence state.
 13. A methodaccording to claim 12 wherein said polyvalent metal compound is acompound of chromium wherein the valence of the chromium is +6 and thevalence of at least a portion of said chromium is reduced to +3.
 14. Amethod according to claim 13 wherein: said chromium compound is selectedfrom the group consisting of ammonium chromate, ammonium dichromate, thealkali metal chromates and dichromates, chromium trioxide, and mixturesthereof.
 15. A method according to claim 14 wherein said polymer is asubstantially linear polymer of acrylamide.
 16. A method according toclaim 15 wherein: said reducing agent is selected from the groupconsisting of hydroquinone, sodium sulfide, hydrogen sulfide, sodiumhydrosulfite, sodium metabisulfite, potassium sulfite, sodium bisulfite,potassium metabisulfite, sodium sulfite, sodium thiosulfate, ferroussulfate, ferrous chloride, p-hydrazinobenzoic acid, hydrazine phosphite,hydrazine dihydrochloride, and mixtures thereof.
 17. A method accordingto claim 12 Wherein: said polymer is a substantially linear polymer ofacrylamide; said polyvalent metal compound is sodium dichromate; andsaid reducing agent is sodium hydrosulfite.
 18. A method according toclaim 11 wherein: said polymer is a substantially linear polyacrylamidewherein from 0.1 to about 12 percent of the carboxamide groups thereinare hydrolyzed to carboxyl groups; and the initial total dissolvedsolids content of said brine is greater than about 60,000 ppm by weight.19. A method according to claim 18 wherein: said brine is thickened byadding thereto from 0.01 to 1.5 weight percent of said polymer, based onthe weight of said brine; and gelation of said thickened brine is causedby adding thereto from 0.5 to 30 weight percent of said polyvalent metalcompound based on the weight of said polymer, and from 0.1 to at leastabout 150 weight percent of the stoichiometric amount of said reducingagent required to reduce said polyvalent metal to said lower valencestate.
 20. A method according to claim 19 wherein said polyvalent metalcompound is a compound of chromium wherein the valence of the chromiumis +6 and the valence of at least a portion of said chromium is reducedto +3.
 21. A method according to claim 20 wherein: said reducing agentis selected from the group consisting of hydroquinone, sodium sulfide,hydrogen sulfide, sodium hydrosulfite, sodium metabisulfite, potassiumsulfite, sodium bisulfite, potassium metabisulfite, sodium sulfite,sodium thiosulfate, ferrous sulfate, ferrous chloride,p-hydrazinobenzoic acid, hydrazine phosphite, hydrazine dihydrochloride,and mixtures thereof.
 22. A method according to claim 19 wherein: saidpolyvalent metal compound is sodium dichromate; and said reducing agentis sodium hydrosulfite.